Examples of self-centering grinding arrangements intended for grinding the tungsten carbide buttons of button bits of the aforesaid kind are described in Swedish Patent Application No. 8604159-7. The grinding pin of these known grinders executes two distinct forms of movement, i.e. the pin rotates about its own longitudinal axis and also carries out a superposed circuitous or orbital movement. The grinding machine and its drive means are mounted on a column-carried jib arm which can be raised and lowered, up and down the column. The bit to be ground is clamped in a fixture or jig which can be moved freely on an electromagnetic table, when the table is not magnetic. Initially, the fixture in which the bit is clamped is brought into approximate alignment with the grinding machine, whereafter the jib arm is lowered, so as to bring the grinding pin into engagement with the carbide button of the bit to be ground. Since, at this stage, the fixture is able to move freely on the table, it is said that the carbide button will center itself in relation to the grinding pin. Subsequent hereto, the table is brought to its magnetic state, such as to lock the fixture in position on the table.
This known grinding arrangement thus includes a holder arrangement which can be moved in the horizontal plane. A similar grinding arrangement is described and illustrated in Swedish Patent Application No. 8702950-0, in which the bit holder arrangement includes a carriage which runs on a platform and which can be raised and lowered relative to the grinding machine. The grinding machine is attached to the top of a stand or frame and is mounted for limited vertical movement therein.
Such known grinding arrangements, however, do not permit bits to be ground in a rational manner, since only one bit can be clamped in the holder at any one time. It is also doubtful that effective self-centering of the button actually takes place, since the holder and the load carried thereby, i.e. the bit, constitute a heavy and relatively sluggish unit which has to be acted upon by the grinding pin itself, in order to center on the pin. Another problem which is not solved satisfactorily by the known grinders is one of controlling the force at which the grinding pin is intended to bear against each individual button on the bit. In the case of known grinder constructions, it would appear that this problem is solved through the intermediary of the weight exerted by the grinding machine and its journalling devices.